Field of the Invention
The present invention relates generally to infrared optics and, more specifically, to infrared lens elements with gradient optical properties and multi-element infrared imaging lens systems with gradient index optical elements.
Description of the Prior Art
It is common to refer to an optical glass as having a refractive index at a certain wavelength and to describe the shape of the dispersion function using the Abbe number, V (or v)=(nd−1)/(nF−nC), and various partial dispersion values, Px,y=(nx−ny)/(nF−nC), as dictated by the precision of the optical design. Since infrared transmitting glasses often have poor transmission for visible wavelengths, a ‘modified’ Abbe number is used where the visible wavelengths, λF, λd, and λC, are replaced with more suitable infrared wavelengths. Two common examples are the mid-wave infrared (MWIR), where the wavelengths 3, 4 and 5 μm are used and the long-wave infrared (LWIR) where the wavelengths 8, 10 and 12 μm are used to define the MWIR dispersion, V(3-5) (or VMWIR)=(n4−1)/(n3−n5) and LWIR dispersion, V(8-12) or (VLWIR)=(n10−1)/(n8−n12) respectively. While these dispersion parameters describe the wavelength dependent refractive index of IR-transmitting materials sufficiently to aid the selection of materials for a lens design, they lack the precision required for modern high performance optical design software. As a result, the refractive index is also represented in either tabular form (a list of indices at specific wavelengths) or more precisely by Sellmeier coefficients that permit interpolation and extrapolation of refractive index values.
Refractive optical imaging systems typically utilize multiple refractive optical elements to manipulate light and create an image. Commonly, these individual homogeneous optical elements are comprised of different optical materials with different optical properties, including refractive indices, dispersions, or thermo-optic coefficients, in such combinations that attempt to reduce or eliminate problems associated with using a single material, including for example chromatic dispersion, spherical aberration, coma, astigmatism, and thermal drift. For various reasons, including reducing system size, weight and complexity or improving performance and reliability, optical designers may opt to use specialized optical elements, such as gradient index (GRIN) optics. A GRIN optic is a single optical element wherein the optical properties vary in a controlled way within the bulk of the optical element. GRIN optics are limited to primarily visible wavelengths as the methods used in their fabrication are not well-suited to IR transparent materials.
Gradient index (GRIN) optics with radial gradients are typically fabricated using ion exchange and diffusion of ions in a porous body. Go!Foton's SELFOC® product is a commercial example of radial GRIN lens. In the ion-exchange process, an optical blank comprised of an oxide glass with mobile dopant ions is submerged in a hot salt bath for an extended time such that the dopant ions in the blank diffuse through the blank into the bath and ions from the bath diffuse into the blank. This exchange (for example Ag+ for Li+) imparts a continuously varying compositional gradient within the blank and thereby a gradient in the optical properties of the optical element. This process is typically not possible with IR transparent materials, especially those used beyond a wavelength of about 1.6 μm, as ion exchange has not been successfully demonstrated in such materials. Moreover, infrared transmitting glasses heavily doped with alkali ions (Li+, Na+, K+, etc. and the like) are not chemically durable. Furthermore, the thermodynamics of diffusion limit the size of optical elements fabricated via the method under reasonable times to about 10 mm in diameter, which poses a problem for imaging optics in general and IR imaging systems specifically.
GRIN optics with axial gradients are commercially available for visible light, for example LightPath Technologies' Gradium® lenses, and are fabricated by a diffusion process wherein one stacks a series of plates of glasses and heats the assemblage for a time and temperature to diffuse the constituents from plate to plate resulting in an optic blank with a gradient in composition and refractive index. The resulting blank is subsequently cut and polished into a lens shape resulting in an optical element with curved surfaces and an internal gradient of optical properties continuously varying along the direction of the optical axis. Since the gradient is in the direction of the optical axis, and the diffusion takes place entirely within the optical element, the diffusion distances and times can be shorter than those for radial GRIN optics. Diffused axial GRIN optical elements are therefore not subject to the same diameter limitations as ion-exchanged radial GRIN optical elements. Axial and radial GRIN optical elements have different transfer functions and uses, and the former typically must have one or two curved surfaces while the latter often has flat surfaces. The methods employed by the prior art are not suitable for use with IR transmitting glasses in general and chalcogenide glasses in particular, wherein one or more elemental components may be prone to sublimation or out-gassing from the glass, thereby uncontrollably changing the glass composition and properties and forming bubbles, pores, voids, soot or other regions of devitrification or phase separation, which also has a detrimental effect on refractive index, dispersion and other optical properties.
Polymer GRIN lenses with axial, radial and spherical gradients have recently been demonstrated by layering polymer films with different optical properties into a stack and subsequently molding and/or machining the surfaces of the stack. The polymer GRIN optical elements are comprised of multiple polymer films wherein each film is on the order of 50 μm thick and is comprised of many (>1000) nano-layers (<10 nm thick) of alternating polymer compositions such that each film possesses a unique refractive index. The polymer GRIN optical elements possess discontinuous or stair-step gradients in refractive index as the polymers comprising the nano-layers do not undergo a chemical diffusion process. In order to distinguish this type of profile from those of the continuously varying “diffused” GRIN optical elements, we term these “segmented” GRIN optical elements, wherein the gradient may be characterized by the finite thickness or other dimension of the segments (in this case ˜50 μm) and a finite change in refractive index (∂n) or other optical property (in this case ∂n=˜0.0016).
The prior art provides GRIN optics for visible wavelengths but none address the application to materials capable of operating at infrared wavelengths greater than about 1.6 μm, for example infrared transmitting glasses and specifically chalcogenide glasses. The method of the current invention addresses the shortcomings of the state of the art and enables the fabrication of gradient index optical elements capable of operating over a broad range of infrared wavelengths, from about 800 nm to about 18 μm.